This invention relates generally to fluid flow control systems incorporating pinch valves through which fluid carrying conduits are threaded and the valves are operated alternately to act upon said conduits to change the flow conditions therethrough between full flow and cease flow.
More particularly, the invention provides an improved pinch valve for use with a flexible hollow fluid conduit of the type having unitary means capable of being acted upon positively to open same from a compressed closed condition to permit full flow therethrough, said pinch valve being a solenoid operated, snap-in assembly which provides means for acting upon said conduit which enables improved response and reduces wear on the conduit, which is less expensive to manufacture and is easier to assemble and to mount compared to prior pinch valve constructions and, as well, is protected against exterior originated contamination.
The art has provided many different types of apparatus to perform automated analysis, many types involving the transfer of liquid in such a manner as to provide quantitatively accurate dilutions and physical transfer of precise volumes of liquid from one location to other locations.
Difficulties were encountered in switching from one flow path to another. One transfer system involved the use of check valves which are normally one-way direction flow valves. Such valves are subject to sealing or seating problems which may be caused by build-up sediment on the sealing mechanism. Thus, error is introduced due to the extra flow past the sealing location occasioned by slow response to change in the condition. Other problems were encountered, such as backlash occuring due to time delay between pressure change to close the valve and the time the valve actually is closed. More liquid than desired may be permitted past the valve before it has the opportunity or time within which fully to close off flow. Swelling was another problem encountered with many check valve constructions. Goals such as instantaneous response were not reached.
In U.S. Pat. Nos. 3,882,889 and 3,932,065, a solution was offered to many of the above mentioned problems. There was provided a pneumatically operated pinch valve of the make-before-break type which operated upon fluid paths defined by flexible conduits. A valve shell was provided which was formed by assembling several plastic molded parts, said shell also being provided with piston means, namely a pair of plastic molded pistons assembled within the shell and slidable linearly therewithin under spring bias. Windows were defined by the shell members to enable passage therethrough of flexible conduits across the path of the pistons. A post also was provided on one of the shell parts extending across the width of the shell and was disposed between the pistons, said post cooperating with at least one of the pistons to enable a conduit to be compressed, stopping flow therethrough. Notwithstanding the substantial benefit providedd by these patent pinch valves and the liquid transfer systems employing same, problems were encountered which were yet to be addressed.
One of these problems involved one of the characteristics of the elastomeric tubing employed as the conduits. This type of tubing possesses a memory to maintain its tubular configuration so as not to interfere with passage therethrough. Such memory normally causes the tubing to return to its open condition after it has been compressed closed, as by pinching a localized section thereof. However, when compressed for an extended period of time, the tubing loses its memory and fails quickly to recover to its fully open condition after prolonged compression. This phenominon may be due to cold flow and often is unpredictable and contributes to slow or uncertain response occurances. The tube can be stuck closed due to the nature of the liquid being transported. Manual override means have been utilized to permit the conduit to be maintained in open condition so that the valve could be rendered inoperative without compressing the conduit. Such additional provisions increase fabrication and assembly cost and do not meet occasions where the conduits are desired to be maintained compressed closed yet to be instantaneously responsive when rendered operative to enable flow.
The problem of restoring the flexible conduit from its compressed condition was considered with the solution thereto offered by the invention disclosed in U.S. Pat. No. 4,653,719. Here, a molded flexible conduit was provided with unitary diametrically opposed longitudinal ear formations extending outwardly of the conduit. A pinch valve was provided for accommodating the modified flexible conduit, said pinch valve having a structure similar to the earlier mentioned pinch valves but having slot means formed in one of the pistons and in the stationary post, each ear formation of the modified flexible conduit being drawn through said slot means to extend across the path of said one piston to enable the piston to compress the conduit against said stationary post in one condition of the valve and to effect a pulling force upon the ear formations simultaneously to enable the conduit to be pulled in opposite directions after the conduit has been compressed, either momentarily or subsequent to a prolonged compressed state. Attention should be directed to the use of two flexible conduits in the systems concerned, two conduits being threaded through the valve, one being normally open and the other normally closed. The said valves are operated first to close the normally open conduit, then to proceed to a state where both conduits are closed and then, finally, to a state where the normally compressed conduit is opened by causing force to be applied to both the ear formations. Only one of the flexible conduits need possess the ear formations.
Improvement in the purchase afforeded to both ear formations which are provided on the patented flexible conduit has been sought so as to improve response time and reduce stress placed on the ear formations which could result in tearing thereof. The nature of the purchase provided by like slots formed in the stationary and movable elements of the pinch valves was likely to result in greater stress applied to the cross-bar of the ear formation than to the juncture of the radial portion thereof with the conduit wall. The cross-bar portion may slip within the slot as the force is applied by pulling upon the opposite ear by the piston through which the formation is threaded. The invention herein contemplates improving upon the prior structure by providing a construction which will direct the restraining force to a location close to the juncture of the radial portion of the ear formation with the conduit wall. In this event, the pulling force may be more effective on the conduit itself to effect full opening thereof and is likely to improve the response time.
Another problem to be considered is the possible contamination of the interior portions of the aforementioned pinch valve from the exterior thereof and attributable to the open construction of the aforementioned patented pinch valve assemblies.
The valve body or housing of the aforementioned prior pinch valve assemblies is formed as an assembly of plural molded parts to form the somewhat open configuration thereof. Since the freedom of movement of the pistons in the relatively open housing is essential, possible entry of debris from the exterior of the housing may interfere with the motion of the pistons or the operation of the exposed spring. Debris may take the form of lint, of solidifying liquids, of adhesive-like materials, gells or the like from spills or other sources. Accordingly, the reduction of open access to the interior moving parts of the pinch valve is desirable but as yet has not been available. The invention contemplates providing a housing for the pinch valve assembly to which casual access is denied.
The above mentioned patented pinch valve assemblies are constructed to operate on a pair of conduits and may not be suitable for use in respect of only one conduit, a portion of the valve being redundant when only a single conduit is employed. Provision of a pinch valve construction which is operable where only a single conduit is employed yet which is at least as effective as the prior patented pinch valves to which reference has been made also is desirable.